Semiconductor transistors, in particular field-effect controlled switching devices such as a MISFET (Metal Insulator Semiconductor Field Effect Transistor), in the following also referred to as MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a HEMT (high-electron-mobility transistor) also known as heterostructure FET (HFET) and modulation-doped FET (MODFET) are used in a variety of applications. An HEMT is a transistor with a junction between two materials having different band gaps, such as GaAs and AlGaAs, and GaN and AlGaN.
HEMTs are viewed as an attractive candidate for microwave and power transistor applications, i.e. applications in which switching of substantially large voltages and/or currents is required. HEMTs offer high conduction and low power losses in comparison to conventional silicon based devices.
HEMTs are commonly formed from III-V semiconductor materials, such as GaAs, GaN, InGaN, AlGaN, etc. In a GaN/AlGaN based HEMT, a two-dimensional electron gas (2DEG) arises at the interface between the AlGaN barrier layer and the GaN buffer layer due to spontaneous and piezoelectric polarization. The 2DEG forms the channel of the device instead of a doped region. Similar principles may be utilized to select buffer and barrier layers that form a two-dimensional hole gas (2DHG) as the channel of the device. A 2DEG or a 2DHG is generally referred to as a two-dimensional carrier gas. Without further measures, the heterojunction configuration leads to a self-conducting, i.e., normally-on, transistor. Measures must be taken to prevent the channel region of an HEMT from being in a conductive state in the absence of a positive gate voltage.
One drawback of type III-V semiconductor HEMTs in comparison to conventional MOSFET devices is the lack of protection mechanism for accommodating large voltage spikes. A conventional MOSFET device includes an intrinsic body diode by virtue of the p-n junction between the drain and body of the device. This intrinsic body diode is antiparallel to source-drain, and provides a leakage path at reveres bias and high transient voltage situation of the device. By contrast, a type III-V semiconductor HEMT does not include a corresponding diode. It utilizes the high resistive wide band gap semiconductor materiel, such as GaAs or GaN, as the insulation between source and drain. As a result, the highly resistive materiel in the drain and drift region and the gate and field plate form capacitors. At high reveres bias, high transient voltage pulse, ESD (electrostatic discharge), or other high voltage situations, the capacitors can be damaged and catastrophic breakdown can occur. This breakdown is the leakage current or hot carrier cumulated result. This is a primary failure mechanism of HEMTs.